"For the ions to acquire a high
energy from a small bunch of electrons
the bunch must move like the carrot in
front of the donkey but also accelerate
to keep pace with the ions.
This is the mode of acceleration called ion drag. Because of the complexity of the motions and interactions I like to think of another analogy.
The ion is like a marble in a saucer on a railway train. Given a sufficiently gradual acceleration the marble will stay in the saucer and acquire the velocity of the t~rai "